There has recently been an increased interest in the use of ambient light sensors, e.g., for use as energy saving light sensors for displays, for controlling backlighting in portable devices such as cell phones and laptop computers, and for various other types of light level measurement and management. Additionally, for various reasons, there is an interest in implementing such ambient light sensors using complementary-metal-oxide semiconductor (CMOS) technology. First, CMOS circuitry is generally less expensive than other technologies, such as Gallium Arsenide or bipolar silicon technologies. Further, CMOS circuitry generally dissipates less power than other technologies. Additionally, CMOS photodetectors can be formed on the same substrate as other low power CMOS devices, such as metal-oxide semiconductor field effect transistors (MOSFETs).
FIG. 1 shows a cross section of a conventional CMOS light sensor 102, which is essentially a single CMOS photodiode, also referred to as a CMOS photodetector. The light sensor 102 includes an N+ region 104, which is heavily doped, and a P− region 106 (which can be a P− epitaxial region), which is lightly doped. All of the above is likely formed on a P+ or P++ substrate, which is heavily doped. It is noted that FIG. 1 and the remaining FIGS. that illustrate light sensors are not drawn to scale.
Still referring to FIG. 1, the N+ region 104 and P− region 106 form a PN junction, and more specifically, a N+/P− junction. This NP junction is reversed biased, e.g., using a voltage source (not shown), which causes a depletion region around the PN junction. When light 112 is incident on the photodetector 102 (and more specifically on the N+ region 104), electron-hole pairs are produced in and near the diode depletion region. Electrons are immediately pulled toward N+ region 104, while holes get pushed down toward P− region 106. These electrons (also referred to as carriers) are captured in N+ region 104 and produce a measurable photocurrent, which can be detected, e.g., using a current detector (not shown). This photocurrent is indicative of the intensity of the light 112, thereby enabling the photodetector to be used as a light sensor.
A problem with such a conventional photodetector is that it detects both visible light and non-visible light, such as infrared (IR) light. This can be appreciated from the graph in FIG. 2, which illustrates an exemplary spectral response of a human eye. Notice that the human eye does not detect IR light, which starts at about 800 nm. Thus, the response of a conventional photodetector can significantly differ from the response of a human eye, especially when the light 112 is produced by an incandescent light, which produces large amounts of IR light. This provides for significantly less than optimal adjustments where such a sensor 102 is used for adjusting backlighting, or the like.
There is a desire to provide light sensors that have a spectral response closer to that of a human eye. Such light sensors can be used, e.g., for appropriately adjusting the backlighting of displays, or the like.